Armada - by Ernest Cline
Zack Lightman has spent his life dreaming. Dreaming that the real world could be a little more like the countless science-fiction books, movies, and videogames he’s spent his life consuming. Dreaming that one day, some fantastic, world-altering event will shatter the monotony of his humdrum existence and whisk him off on some grand space-faring adventure.

But hey, there’s nothing wrong with a little escapism, right? After all, Zack tells himself, he knows the difference between fantasy and reality. He knows that here in the real world, aimless teenage gamers with anger issues don’t get chosen to save the universe.

And then he sees the flying saucer.

Even stranger, the alien ship he’s staring at is straight out of the videogame he plays every night, a hugely popular online flight simulator called Armada—in which gamers just happen to be protecting the earth from alien invaders.